PCR clock recovery in an IP network

ABSTRACT

An IP network includes a central entity and at least one customer premises equipment (CPE) device. The central entity generates a program clock reference (PCR) clock and provides audio-visual packets to a CPE based on the PCR clock. The CPE sets a first clock based on the PCR clock for decoding operations. The CPE sets a second clock that is independent from the first clock for audio and video output operations. For example, the CPE can process the audio-visual packets using the second clock.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit to U.S. Provisional Patent ApplicationNo. 60/812,087, filed Jun. 9, 2006, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The inventions relate generally to clock recovery, and more specificallyto program clock reference (PCR) clock recovery in an internet protocol(IP) network. The inventions apply even more generally to audio andvideo time management, clock control and display clock control. PCRclock recovery is an important component enabling a decoder clock to besynchronized with the encoder clock in a point to multipoint broadcastnetwork.

In point-to-multipoint communication systems, an IP network supportsbidirectional data communication between a central entity and multiplecustomer premises equipment (CPE). Example point-to-multipointcommunication systems include cable modem systems, fixed wirelesssystems, and satellite communication systems. In each system, thecommunication path from the central entity to the CPE is typicallyreferred to as the downstream, while the communication path from the CPEto the central entity is typically referred to as the upstream. A CPEmay be a cable modem, a settop box, or a cable gateway, to provide someexamples.

Audio-visual information may be transferred in an EP network inaccordance with any of a variety of standards, such as the InternationalOrganization for Standardization/International ElectrotechnicalCommission 13818-1 International Standard, published on Nov. 13, 1994(the ISO/IEC 13818 standard). This standard is consistent with MPEG2.The central entity of the point-to-multipoint communication systemgenerates a program clock reference (PCR) clock in accordance with thestandard and transmits the audio-visual information based on the PCRclock. The CPE(s) traditionally processes the audio-visual informationfor display using the PCR clock. However, audio-video information in IPnetworks often exhibits relatively large and irregular propagationdelays, hindering the CPE(s) from adequately recovering the PCR clock.Moreover, PCR timestamps may not be sufficiently reliable for PCR clockrecovery.

What is needed, therefore, is a system and method that addresses one ormore of the aforementioned shortcomings of conventional PCR clockrecovery techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate embodiments of the present inventionand, together with the description, further serve to explain theprinciples of the invention and to enable a person skilled in thepertinent art(s) to make and use the invention.

FIG. 1 is a block diagram of an example IP network.

FIG. 2 is another block diagram of the example IP network shown in FIG.1, according to an embodiment of the present invention.

FIG. 3 is a block diagram of an example IP network having first andsecond clocks, with the second clock being a fixed crystal referenceclock.

FIG. 4 is a simplified block diagram showing an example of transportpaths of a BCM7401 chip, according to embodiments of the presentinvention.

FIG. 5 illustrates an example Ethernet frame, according to an embodimentof the present invention.

FIG. 6 is a flowchart 600 of a method of providing packets to a CPE inaccordance with an embodiment of the present invention.

FIG. 7 shows a plot of PCR/STC with reference to time for an off-airbroadcast, according to an example embodiment of the present invention.

FIG. 8 shows a plot of PCR/STC with reference to time for an IPmulticast, according to an example embodiment of the present invention.

In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the leftmost digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

DETAILED DESCRIPTION OF THE INVENTION

Although the embodiments of the invention described herein referspecifically, and by way of example, to point-to-multipointcommunication systems and components thereof, including settop boxes, itwill be readily apparent to persons skilled in the relevant art(s) thatthe invention is equally applicable to other devices and systems. Itwill also be readily apparent to persons skilled in the relevant art(s)that the invention is applicable to any apparatus or system requiringPCR clock recovery.

This specification describes one or more embodiments that incorporatethe features of this invention. The embodiment(s) described, andreferences in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment(s) describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Furthermore, when a particularfeature, structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

Overview

Conventional PCR clock recovery logic assumes a maximum network delay ofapproximately 2 milliseconds (ms), though delays of as large as 300 msmay be encountered in an IP network. Analog video outputs are sensitiveto timebase variations, making it difficult to compensate for delays inclocking information quickly enough to avoid problems in the video andaudio outputs of a CPE which often have very sensitive timingrequirements. Avoiding these issues using conventional techniquesrequires extensive buffering of audio and video data. This bufferingsubstantially increases channel change times in the IP network.

When decoders and outputs utilize the same timebase, even relativelysmall adjustments in the timebase can cause undesired effects and/ordisturbances in video and audio outputs. By decoupling the inputtimebase used by the decoders from the output timebase used by the audioand video outputs, these effects can be avoided or substantiallyreduced. This allows for coarse adjustments in the decoder clocks torapidly respond to network jitter conditions while allowing for moregradual or no adjustment of the clocks used by the CPE to output audioand video signals.

Example PCR Clock Recovery Embodiments

FIG. 1 is a block diagram of an example IP network 100. In FIG. 1, IPnetwork 100 includes a central entity 102 and a CPE 104. This particularexample relates to a video distribution network. Central entity 102 is abroadcast network and CPE 104 is a settop box (settop decoder). The useof a video distribution network, here and later in this patent documentand the labeling in the figures of central entity 102 and CPE 104 as“settop decoder” is not intended to limit the scope of the claimedinventions, but merely to provide an example to the reader. Personsskilled in the relevant art(s) will recognize that central entity 102and CPE 104 may be any of a variety of devices/systems.

Central entity 102 includes an encoder 106 that encodes audio-visualpackets according to a standard, such as, for example, the ISO/IEC 13818standard (various standards are applicable for various types ofsystems). The ISO/IEC 13818 standard specifies a maximum allowable PCRspacing to facilitate proper PCR clock recovery at CPE 104. For example,the ISO/IEC 13818 standard specifies a PCR spacing of less than 100 ms.

Annex D of the ISO/IEC 13818 standard defines a well behaved system asone exhibiting less than 4 ms of network induced timing delay (i.e.,jitter). In IP video streaming, it is common to observe jitter thatexceeds these constraints (e.g. >300 ms). The situation can be furthercomplicated by software induced jitter introduced while processing theIP video packets and/or while providing the payload to CPE 104 forde-multiplexing, decryption and decoding. ISO/IEC 13818 Annex Ddescribes the timing model used in digital broadcast networks and theimplications of relatively large network delay and PCR jitter. ISO/IEC13818 Annex D further describes the need for a CPE 104 to have aconsistent output clock to adhere to typical analog video display timingrequirements.

In the FIG. 1 video distribution network example, CPE 104 includes aphase-locked loop (PLL) 108, a settop clock 110, a compressed buffer112, an audio-video (A/V) decoder 114, an audio-video display 116, and aframe buffer 118. The encoded audio-visual packets received from centralentity 102 are processed by PLL 108 and compressed buffer 112. PLL 108generates/recovers settop clock 110 from PCRs in the transport stream.A/V decoder 114 uses settop clock 110 to decode the audio-visual packetsthat are buffered by compressed buffer 112. Frame buffer 118 buffers thedecompressed/decoded audio-visual frames received from A/V decoder 114.A/V display 116 generates an analog output using settop clock 110 andthe decompressed/decoded frames provided by frame buffer 118.

In typical broadcast decode settop devices, PLLs are not designed tohandle large PCR jitter because of the timing requirements of analogdisplay standards, such as those set forth by the National TelevisionSystem Committee (NTSC). Accordingly, in FIG. 1, A/V decoder 114 and A/Vdisplay 116 share the same (or a tightly coupled) settop clock 110,which is corrected by PLL 108. In the embodiment of FIG. 1, it may besaid that display timing “tracks” decode timing. Sections 0.4-0.6 ofAnnex D discuss this issue in greater detail.

One technique to compensate for relatively large delays and/or PCRjitter while still adhering to the rigid analog display timingrequirements described above is to decouple the decoder clock from thedisplay clock. FIG. 2 is another block diagram of the example IP networkshown in FIG. 1, according to an embodiment of the present invention.

In FIG. 2, CPE 104 includes a first clock 210 a, a second clock 210 b,and a clock generation module 220. First clock 210 a and second clock210 b are labeled as a decoder timebase and a display timebase,respectively, for illustrative purposes. Clock generation module islabeled as a software control for illustrative purposes and is notintended to limit the scope of the present invention. Clock generationmodule 220 may include software, hardware, firmware, or any combinationthereof.

PLL 108 generates/recovers first clock 210 a based on PCRs in thetransport stream. A/V decoder 114 uses first clock 210 a to decode theaudio-visual packets that are buffered by compressed buffer 112.However, in FIG. 2 a, A/V display 116 does not use first clock 210 a togenerates the analog output. Instead, A/V display 116 generates theanalog output using second clock 210 b. Second clock 210 b is notgenerated based on PCRs in the transport stream. Instead, clockgeneration module 220 sets second clock 210 b. Second clock 210 b isgenerated independently from first clock 210 a. Second clock 210 b mayhave a different frequency and/or phase than first clock 210 a.

Decoupling first clock 210 a, which is associated with A/V decoder 114,and second clock 210 b, which is associated with A/V display 116, allowsfor second clock 210 b to adhere to the stringent analog display timingrequirements but allows for more course adjustment of A/V decoder 114.By employing techniques in A/V decoder 114 to drop, repeat, andinterpolate decoded frames, fairly course adjustments of first clock 210a can be masked. A substantially slower adjustment of second clock 210 badhering to the output timing requirements can be employed to keep firstclock 210 a and second clock 210 b loosely synchronized. In anembodiment, network 100 performs sync-slip operations in the displaypipeline to handle a plurality of display source and output formats.Decoupling first and second clocks 210 a-b will utilize this establishedbehavior to avoid underflows and overflows in the display pipelinecaused by the loosely synchronized first and second clocks 210 a-b.

Any of a variety of techniques may be used to generate second clock 210b. In an example embodiment, shown in FIG. 3, second clock 210 b is afixed crystal reference clock. Unlike first clock 210 a, the fixedcrystal reference clock does not track the PCR clock that is received byCPE 104. The analog output provided by A/V display 116 may sync-slip,meaning that the frame rate of a window does not match the frame rate ofA/V decoder 114. Sync-slipping causes the window to skip or repeatframes.

Assuming for illustrative purposes that the video source and the crystalused to generate second clock 210 b are each accurate within +/−60parts-per-million (ppm), the total difference would be at most 120 ppm,corresponding with a maximum sync-slip of 120/1000000*30 frames/sec*60sec/min*60 min/hour=13 frames/hour. Based on these assumptions, async-slip would occur on average once every 4.5 minutes, worst case.

In this embodiment, using the fixed crystal reference clock for displaytiming necessitates setting the display timebase for each output to afixed value. The method for fixing the display clock or timebase mayvary from system to system. Some systems support dual-decode anddual-display, e.g. “Picture in Picture”. Such systems may support voiceover Internet protocol (VoIP) on one display, but not the other. Thesesystems may need to use a fixed timebase for one display, but not forthe other.

FIG. 4 is a simplified block diagram showing, as an example, transportpaths of a BCM7401 chip (“7401 embodiment example”, or “7401”),manufactured by Broadcom Corporation, according to embodiments of thepresent invention. This chip is suitable for use, for example, in asettop box in a video distribution network.

In this example embodiment, which actually implements a generalizationof the first example, first clock 210 a and second clock 210 b are basedon different timebases. Software control 720 is used to adjust thesecond clock to speed up or slow down the display timebase within thetolerances of the display. The first timebase is used for decode timing,and the second timebase is used for display timing. Audio-visualdecoders, such as A/V decoder 114, reference the first timebase. Displayoutputs, such as A/V display 116 reference the second timebase.

For normal (non-VoIP) broadcasts, the first and second timebases bothare locked to the incoming stream, such that the system behavesnormally, using PCR values in the input stream. This is possible becausethe PCR values are reliable and will not cause timing problems thedisplay outputs. For VoIP broadcasts with large jitter and unreliableclock information, only the decoder timebase which can tolerate coarsetimebase adjustment can be corrected or adjusted using PCR values in theincoming stream. The second timebase is locked to the fixed crystalreference clock or adjusted by software in a more controlled fashion.This is important because display or output timebases must transitionrelatively slowly or suffer video artifacts on the display outputs.

Network jitter and delays in a VoIP network can be larger than thoseseen in typical broadcast networks (i.e. 300 ms vs 4 ms) and IP softwareprotocol stacks in the settop decoder can introduce additionalprocessing delay. In this 7401 embodiment example the followingadjustments are made to handle these conditions:

-   -   Decouple decoder and display timebases (clocks). The 7401 has        two separate timebase controls as well as the ability to fix the        display or decoder clock frequencies.    -   Increase amount of data in the decoder compressed buffer to        compensate for additional network delay. For example if the        maximum network delay is 300 ms extra of data must be buffered        to insure the compressed data buffer never becomes empty. This        is accomplished on 7401 by delaying decode by 300 ms.    -   Decrease PLL sensitivity because PCR values are less reliable        and tend to exceed conventional thresholds. On the 7401 the PCR        discard threshold can be configured by software to account for        the large maximum delay or jitter in the network.    -   Allow for more course adjustments in decoder clock which is        possible because with a decoupled decoder and display clock the        decoder can tolerate these course adjustments. The 7401 decoder        timebase control can be configured to accept larger or more        coarse adjustments.    -   Prevent software processing delays in the CPE settop decoder by        implementing an Ethernet injector 702 (also see FIG. 6        functional flowchart) which utilizes DMA (Direct Memory Access)        hardware to “inject” audio/video data directly into a transport        demultiplexor 704 as if the IP audio/video data were received        from a traditional broadcast digital network        receiver/demodulator.

Decoupling first clock 210 a and second clock 210 b enables CPE 104 toperform a faster channel change, as compared to conventional techniquesfor handling large network delays. For example, conventional decodersrequire more buffering because they cannot cope with the largediscontinuities or delays in an IP network and therefore sometimesutilize the PTS (presentation time stamp) in the audio/video stream toconfigure the local decoder and display timebase. Typically thistechnique adds a half second or more to the channel change time becausemore data must be buffered before a valid PTS is observed by the decoderand used to program the decoder timebase.

FIG. 5 illustrates an example Ethernet frame, according to an embodimentof the present invention. The physical interface for IP network 100 isEthernet, and video stream packets are segmented to fit within a singleEthernet frame 300, as shown in FIG. 4. As shown in the figure, a packetincludes an Ethernet MAC Header, an IP Header, a UDP Header, a RTPHeader (optional), a Transport Packet Header, and seven (7) transportpackets. Audio and video is encapsulated in the transport packets.Software induced jitter may be mitigated by providing Ethernet A/Vpayload directly to transport demuliplexor (in 7401 figure above), forexample.

FIG. 6 is a flowchart 600 of a method of injecting packets to a CPE inaccordance with an embodiment of the present invention. The invention,however, is not limited to the description provided by flowchart 600.Rather, it will be apparent to persons skilled in the relevant art(s)from the teachings provided herein that other functional flows arewithin the scope and spirit of the present invention.

Flowchart 600 will be described with continued reference to the BCM7401chip described above, though the method is not limited to thisembodiment. In this preferred embodiment the Ethernet injector carriesout all of the steps shown in FIG. 6.

Referring now to FIG. 6, the CPE receives a packet at block 610. If thepacket is a video packet, as determined at decision block 620, then aplayback descriptor is created at block 630. For example, a payloadoffset may be calculated e.g. by Ethernet injector shown in FIG. 4 intoeach Ethernet frame, and the playback descriptor is assigned to feed thepayload to the transport demux of the BCM7401 chip. The packet isreturned for reuse at block 640, and control returns to block 610. Onthe other hand, if the packet received at block 610 is not a videopacket, as determined at decision block 620, then control returns toblock 610.

In the embodiment of FIG. 6, IP packets are filtered and then provideddirectly to the BCM7401 chip via the transport playback without copyingor intermediate buffering, which may reduce channel change time, reduceCPU overhead, and/or reduce software induced jitter, to provide someexamples. This method of injecting packets may minimize systemcomplexity while substantially reducing software induced jitter.

In broadcast video networks, jitter is minimal and PCRs arrive at aprecise rate. FIG. 7 shows a plot 700 of PCR/STC with reference to timefor an off-air broadcast, according to an example embodiment of thepresent invention. In the embodiment of FIG. 7, PCRs are shown to arriveat least every 100 ms. However, the scope of the present invention isnot limited in this respect.

IP networks generally are not well behaved, even in a controlledlaboratory environment. When PCR clock recovery logic used inconventional broadcast networks is used for an IP delivered stream it isnot uncommon for the decoders to exhibit problems, such as A/V decoder114 shown in FIGS. 1 and 2, will periodically fall in an out of lockbecause the default configuration of the PCR clock recovery logicidentifies PCR discontinuities in the IP delivered stream. In the 7401this behavior can be monitored to develop optimized thresholds andbuffering to handle the delay and jitter present in a particular IPnetwork.

The following Table A represents an example embodiment showing PCRdiscontinuities after starting an IP stream decode. In this embodiment,using typical broadcast thresholds in an IP network the behaviordescribed in 0039 is observed. The value “1” in the pcr_invalid columncoincides with a disruption in both audio and video decode.

TABLE A Usec Pcr pcr_invalid load 0 7447953 0 0 56 7447953 0 1 282407452005 1 0 57131 7456050 1 0 132983 7460103 0 0 133082 7460103 0 1232901 7464156 1 0 322910 7468201 1 0 393001 7472254 0 0 393100 74722540 1 482903 7476314 0 0 511507 7480351 1 0 662993 7484404 1 0 7528977488457 0 0

In embodiments, network induced jitter of 300 ms and minor data errorscan cause the decoder compressed data buffers (CDB), such as compressedbuffer 112 shown in FIGS. 1 and 2, to underflow. To compensate, thedepth of the audio and video CDB is increased by providing a “displayoffset” for video and an “A/V offset” for audio. Utilization of thedisplay offset and the A/V offset delays the decoders, allowing the CDBsto run much deeper and increasing the resilience to network jitter. Theconfiguration for the respective offsets and the CDB size is dependentupon the network environment.

To validate using the BCM7401 hardware for clock recovery, a testprogram may be used to intercept and rebroadcast multicast IP videostreams, intentionally introducing periodic jitter. This has the effectof stopping data flow for up to 300 ms, for example, then delivering thedelayed data at a bit rate substantially higher than the average streambit rate until a steady state is again reached.

FIG. 7 depicts the steady arrival of PCRs in a typical broadcast networkover a sample period of time. FIG. 8 is a graphical representation 900of PCR/STC with reference to time for an IP multicast, according to anexample embodiment of the inventions. The PCR/STC values correspond toarrival times of the PCRs. A 300 ms discontinuity is shown in FIG. 8 atapproximately 2.5 sec into the decode. As shown, the PCR values bunch upafter the discontinuity.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Moreover,firmware, software, routines, instructions, etc. may be described hereinas performing certain actions. However, it should be appreciated thatsuch descriptions are merely for convenience and that such actions infact result from computing devices, processors, controllers, or otherdevices executing the firmware, software, routines, instructions, etc.

CONCLUSION

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A customer premises equipment (CPE) device configured to receiveaudio-visual packets associated with a program clock reference (PCR)clock, comprising: a phase-locked loop (PLL) to set a first clock basedon the program clock reference clock; a second clock that is independentfrom the first clock; an audio-visual decoder to decode the audio-visualpackets based on the first clock; and an audio-visual display togenerate an analog signal based on the audio-visual packets and thesecond clock.
 2. The CPE device of claim 1, wherein the second clock isbased on a fixed crystal reference clock.
 3. The CPE device of claim 1,further comprising: a clock generation module to set the second clock.4. The CPE device of claim 1, further comprising: a buffer coupled to aninput of the audio-visual decoder to buffer the audio-visual packets. 5.The CPE device of claim 1, further comprising: a buffer coupled betweenthe audio-visual decoder and the audio-visual display to buffer theaudio-visual packets.
 6. In a customer premises equipment (CPE) device,a method of processing audio-visual packets, comprising: generating afirst clock based on a program clock reference (PCR) clock; decoding theaudio visual packets based on the first clock; generating a second clockthat is independent from the first clock; and generating an analogsignal based on the audio-visual packets and the second clock.
 7. Acustomer premises equipment (CPE) device configured to receiveaudio-visual packets associated with a program clock reference (PCR)clock, comprising: means for generating a first clock based on a programclock reference clock; means for decoding the audio visual packets basedon the first clock; means for generating a second clock that isindependent from the first clock; and means for generating an analogsignal based on the audio-visual packets and the second clock.
 8. TheCPE device of claim 7, wherein the means for generating the second clockis based on a fixed crystal reference clock.
 9. The CPE device of claim7, wherein the means for generating a second clock includes a clockgeneration module to set the second clock.
 10. The CPE device of claim7, further comprising: a buffer coupled to an input of the audio-visualdecoder to buffer the audio-visual packets.
 11. The CPE device of claim7, further comprising: a buffer coupled between the audio-visual decoderand the audio-visual display to buffer the audio-visual packets.